Trojans finished third in the Investec Women’s Western Conference, two points behind Gloucester City, who were runners-up to runaway champions Stourport.
A 15th minute opener by Heather Batten and teenager Imo Lea’s second goal midway through the second half secured a 2-0 win over Barnes.
Gloucester City, meanwhile, lost 3-1 at Stourport, who now go into the promotion play-offs alongside Hampstead & Westminster, Loughborough Students and Slough.
Southampton ladies will also be in promotion play-off action against Amersham & Chatfont to decide to progresses beyond Division 3 in the South Women’s League.
They pair finished third in the respective Division 3 A and B Divisions, Southampton rounding off their fixtures with a 3-0 win at Tulse Hill & Dulwich, where Naomi Bulkeley, Lou Hopkins and Zoe Stride were on target.
Southampton’s play-off is at Basingstoke on Saturday April 13.
Fareham had to settle for a fourth place finish after losing their last game 3-2 to visiting Lewes.
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